Plastic materials have been widely used in various optical applications, particularly in manufacturing spectacle lenses, because of their lightweight, toughness and easiness of dyeing. Optical materials, particularly spectacle lenses are required to have, in addition to a low specific gravity, a high clearness, a low yellowness, and optical properties such as a high refractive index and a large Abbe's number. Since a large refractive index reduces the thickness of a lens, many studies have been made thereon to date and many novel compounds have been proposed. Recently, many organic compounds having sulfur atom and/or selenium atom have been proposed to achieve a high refractive index and a high Abbe's number. Of such compounds, some polyepisulfide compounds have been found to be well balanced in refractive index and Abbe's number. For example, Japanese Patent Application Laid-Open No. 9-110979 proposes a straight-chain polyepisulfide compound, Japanese Patent Application Laid-Open No. 9-71580 proposes a branched polyepisulfide compound, Japanese Patent Application Laid-Open No. 9-255781 proposes a polyepisulfide compound having a cyclic structure, and Japanese Patent Application Laid-Open No. 11-140046 proposes a polyepisulfide compound having selenium atom. However, the refractive index of materials obtained from these polyepisulfide are limited to 1.73 at most.